Many construction machines are hydraulically driven. Examples of hydraulically driven construction machines include hydraulic shovels. Generally, in hydraulic shovels, a shovel is driven, an upper-part turning body is caused to turn, and a lower-part traveling body is caused to travel using a hydraulic actuator (hydraulic cylinders and hydraulic motors). In general, hydraulic pressure supplied to a hydraulic actuator is often generated by a hydraulic pump whose drive source is an engine. In this case, the output of the hydraulic actuator is determined by the output of the engine.
Full capacity of the engine is not always required. For example, 90% or 80% of the capacity is often sufficient for the operations of hydraulic shovels. Therefore, the operation mode of the hydraulic shovel is changed according to work loads so as to optimize engine output control for different work loads, thereby efficiently driving the engine to reduce fuel consumption.
For example, it is made possible to set different operation modes such as a “high load mode” for performing a load operation corresponding to the maximum output of the engine, a “normal load mode” for performing a normal-load operation, and a “low load mode” for performing a light-load operation. Then, iso-horsepower control is performed so as to equalize the drive torque required by the hydraulic pump to drive the hydraulic actuator with the output torque of the engine in each of the operation modes, thereby efficiently utilizing the output of the engine to improve fuel consumption.
Generally, a hydraulic shovel has an engine having a maximum output corresponding to an output in the “high load state”. However, a time under the “high load state” is shorter than the “normal load state”. Therefore, there is a sufficient margin in the engine output when the hydraulic shovel is driven with the “normal load state”. Said differently, the hydraulic shovels generally include engines having excessive outputs for the operation under the “normal load state”.
In recent years and continuing, there is a demand for reduction in the amount of fuel consumed by the engine in a hydraulically driven construction machine including the above hydraulic shovel. Simple reduction in the size of the engine would result in insufficient hydraulic output at the time of driving in the “high load mode.” Therefore, so-called hybrid hydraulic shovels have been developed that include an engine, a generator driven by the engine, a battery charged by the generator, and an electric motor driven with the electric power of the battery.